Touch-sensitive screens are placed in proximity to a video screen or cathode ray tube (CRT) for determining, for example, which item of a menu of displayed items has been touched (selected) by the terminal user. The well known Carroll touch screen, disclosed in U.S. Pat. No. 4,267,443, is one such example. The Carroll touch screen uses an array of photodiodes and an array of light sources, such as light emitting diodes, arranged in an x-y matrix to determine which portion or point on the CRT screen is touched by the user.
Other prior art touch-screens use clear compliant material or plate glass as a CRT overlay. These devices rely on the total internal reflection of the CRT raster signal to locate the x and y coordinates of the point on the overlay that is touched by the user. The method of triangulation can also be used to determine the location of the touch point. For example, the Illinois Tool Work's Cyclops touch-sensitive screen uses, among other things, a rotating beam, a beam splitter and a microprocessor which notes the position of the rotating beam when the overlay is touched. It next determines which light beams have been broken by the user's finger and then determines the location of the touch point using triangulation. While this arrangement may be useful, it nonetheless employs mechanical devices, such as the rotating beam, which require frequent adjustments and which tend to fail over time due to fatigue.
It appears that the prior art of touch screens is generally content to just determine the point at which the screen is touched and overlooks other useful information that can be obtained from the force that is applied by the user when touching the screen, for example, whether the applied force is in an upward, downward or sideways direction. Such information could be useful for controlling, for example, the movement of the screen cursor as disclosed in an article appearing in Electronics Week, Volume 57, No. 15, July 31, 1984. The touch-sensitive screen disclosed in this article uses strain gauges, one positioned at each corner of the screen, for determining the location, pressure and shear forces applied to the screen when the user touches it. However, this device also employs mechanical devices, such as strain gauges, which, as mentioned above, tend to be problematical.